1. Field of the Invention
The present invention relates to materials and methods for contacting a solution with a substrate and separating the substrate from the solution. More specifically, the present invention relates to composite micro-sized, magnetic particles for use in extracting desirable or undesirable components from a suspension or solution.
2. Description of the Related Art
Current methods for separating biological materials from impurities and/or suspending media employ the use of very fine magnetic particles that have coated thereon an active separating material. In this context, it is desirable that the magnetic materials possess at least two physical properties, namely, (1) a proper combined mass and size and (2) paramagnetic qualities. The former property will allow the particle to settle more slowly in suspension, thereby enhancing the particles' exposure to and interaction with the suspension. Thus, the smaller they are, the more slowly the particles will settle out according to Stokes Law and the effects of Brownian motion in the case of ultra small particles. The latter paramagnetism property allows the particles to be subsequently removed from the suspension by application of a magnetic field and for example, decanting off the suspending liquid. Since the particles are paramagnetic they will not have had induced residual magnetism and with the field removed, can be re-suspended in yet another recovery medium if necessary without clumping together.
Accordingly, current practice involves the use of very fine paramagnetic particles, consisting of iron oxide and silica composites, some of which are coated and others are mixtures. These particles typically are on the order of 10 to 100 nanometers, for example, and are suspended directly in a solution or suspension containing the nucleic acid or other molecules capable of being extracted from the suspension. Generally, these nano-sized particles contain a coating of an “active” material, that is, a material that has an affinity for a desired material already in suspension or solution. The coated magnetic particles are then separated from the suspension by application of a magnetic field.
Once the desired material in suspension has bound to the active material that is coated on the nano-sized particles, the particles are removed from the suspension. These bound materials can be removed by dissolution with reagents. However, these nano-sized particles often are too minute to separate completely from the suspension. Further, the high surface area of the fine particles increases their own susceptibility to dissolution as well, thus adding an impurity to the extracted media. Thus, a substantial concentration of these particles may remain in suspension and are lost in waste streams. Still further, undesirable clumping may occur when nucleic acid molecules attach to multiple magnetic particles, which are of comparable size, forming chains or large groups of the two. As a result, it is difficult to obtain desirable amounts of material that may have adhered to the particles. For the particles that actually are separated from suspension, multiple successive rinsing steps with extractive solutions are required.
Therefore, there is a present need for larger particles, for example, particles on the order of a sub-micron size to tens of microns, which would perform the function of material removal at high yield and be magnetically separable. By virtue of their size, micro-sized particles meeting these criteria could be separated from suspension more easily than nano-sized particles. Accordingly, the use of these particles would facilitate robotic manipulation of the separation process.
However, an increase in the diameter of these sphere-like particles disproportionately increases their mass, typically resulting in an increased rate of settling out of suspension. Further, agitation such as by stirring to maintain suspension may damage the delicate bio-substances. Hence, there is also a need for a gentle means to keep the particles suspended for times sufficient to allow the desired removal processes to take place.